In film transport systems, it is well known to use a spoolless takeup chamber to temporarily hold the free end of the filmstrip in a compact cylindrical volume. Such chambers are known to employ a curvilinear spring coaxial with the cylinder, the spring having a radius in its relaxed state which is much smaller than the radius of the chamber. When film is initially thrust into the chamber the spring forms the film into a tight coil. With continued insertion of the film, the spring diameter expands as the diameter of the film coil grows. This arrangement allows the film coil to rotate as the inserted film is lapped onto the outer circumference of the film coil thereby substantially eliminating sliding friction between adjacent film surfaces which would cause damage to the film emulsion. The sliding friction would also cause binding of the film in the chamber which would severely limit the ability to thrust any significant length of film into the chamber.
U.S. Pat. No. 5,746,388 discloses a film takeup chamber that utilizes a pair of spaced-apart, expandable, coiled wire springs to form the film into a rotatable film coil with minimal contact between the circumferential film surface and the wire coil. However, the spring wires for this purpose are very small diameter wires which requires that the free ends of the springs be constrained in special slots formed in the takeup chamber walls to avoid twisting and lateral motion of the springs as the film coil grows in diameter. This increases the cost and complexity of the chamber design.
The use of flat springs in spoolless film takeup chambers is known and avoids the need for special constraint slots in the chamber design. A representative example of the use of flat takeup springs in a film takeup chamber is shown in U.S. Pat. NO. 3,288,388. This patent recognizes the problem that as the outer diameter of the film coil increases, the film coil tends to expand and flatten the spring so that a portion of the coil no longer extends along an arc of a circle and, as a result, the spring presses with a non-uniform pressure against the coil of film, deforming the film. The solution proposed in this patent is to add a free wheeling center spool to maintain the film in cylindrical coil form against the uneven spring pressures. This adds undesired cost and complexity to the chamber design and further does not alleviate the problem of excess frictional forces against the film surface caused by the increasing spring forces exerted as the spring flattens out.